1. Field of the Invention
The present invention pertains to air filters. More specifically, the present invention pertains to filter assemblies suitable for removing particulates and undesirable gases from a stream of contaminated air.
2. Description of the Prior Art
For many years air filters have been used to filter particulates from the air which we breathe. Various types of fabric filters have been used to remove pollen, dust, lint, bacteria, and other airborne pollutants from air circulated through heating and cooling systems. Some of these filters are very effective in removing particulates from the air.
In more recent years, air filters have been designed which remove not only airborne particulates, but undesirable gases as well, e.g. gases associated with cooking, tobacco smoking and other gases emitted from humans, other animals or other processes. Such filters are usually designed as combination filters, combining various fabrics for filtering particulates and other materials for sorbing undesirable gases. Materials for sorbing such undesirable gases may include activated charcoal, silica gel, activated alumina, etc.
Early examples of combination filter assemblies for filtering particulates and sorbing gases are shown in U.S. Pat. Nos. 2,544,733 and 2,764,251. In these early filter assemblies, granular sorbents, such as activated charcoal, silica gel, activated alumina, etc. are sandwiched with one or more layers of filter fabric. The fabric is primarily for filtering particulates and the sorbent is primarily for removing undesirable gases.
Of course, the capacity of particulate filtering is, to a great extent, dependent on the surface area of the filter material through which the contaminated air stream flows. The capacity of gas sorption is to a large extent dependent on the volume of the sorbent material in the filter. Thus, the ideal combination filter assembly should have a relatively large surface area for particulates and a suitable volume of sorbent for gases. In reality, the areas and volumes should be specifically designed for the anticipated amounts of contaminant in the air. However, a combination air filter can be designed which is suitable for most household uses. In designing such a filter, care must be taken to make sure that the filter assembly does not induce a large pressure drop. Most of the domestic and commercial air conditioning and heating units have the maximum allowable pressure drop of 0.5 to 1.0 inches of water gauge. These units would not operate efficiently at pressure drops above the maxima.
In U.S. Pat. No. 4,514,197 a filter is disclosed which is provided with a flat or planar screen of mesh material and a corrugated screen having v-shaped corrugations to define a plurality of parallel elongated troughs or voids of triangular shaped cross-section between the two screens in which porous sorbent materials, such as activated charcoal, are deposited. A flat foam prefilter is provided with this design. Apparently, one of the objects of this filter is the provision of greater filtering area and reduced pressure drop. In a somewhat related patent, U.S. Pat. No. 5,129,929, a filter is provided in which there are two major surfaces, the downstream one of which is flat and an upstream surface of which is formed into a plurality of parallel ridges. The voids between these surfaces are essentially triangular in cross-section and are filled with sorbent for filtering undesirable gases from the air passing therethrough. In one embodiment, the upstream surface is provided with a pleated filter fabric to provide greater surface area for particulate filtering.
While the surface area of particulate filtering and the volume of sorbent for gas filtering of U.S. Pat. Nos. 4,514,197 and 5,129,929 offer improvements over the prior art, these filters may not be easily assembled in low cost mass production. In fact, low cost combination particulate and gas filters of high efficiency and low pressure drop do not appear to be readily available. Consequently, the most prevalent product currently available to consumers takes the form of a fibrous mat (felt) coated with charcoal powder. Of course such a filter is lacking in sorbent capacity and must be used in combination with a separate fabric filter to supplement its poor particulate filtering capability. While custom built carbon filtering beds are available from many vendors, they usually require special containers and adapters for use with domestic and commercial air conditioning and heating systems. Furthermore, they are not affordable for most applications.
Obviously, combination particulate and gas filters which are more efficient and more affordable are desired. Such filters would find ready acceptance in the market place.